Service Manager upgrades and 1642 error

Applying a CU for Service Manager and getting "Patcher returned error 1642:Unknown error (0x66a)"?  Odds are you are trying to apply the wrong CU.  For example, you downloaded CU3 for Service Manager 2010 and you really meant to download the CU3 for Service Manager 2010 SP1.  Make sure you are applying the right bits and it should take care of that particular error.